Write neatly.
Don't skip steps in your working out.
Justify each step clearly.
Did you answer the question asked?
Ask yourself if your answer is plausible, is it roughly what you would expect? If not double check. Maybe even guess what the answer may be (or the form the answer should take) before jumping into the question. Use your intuition.
Double check anyway, preferably using a different method (you're not likely to spot a mistake if you use the same method). Use common sense too, if you want to check an integral, differentiate. If you want to check an identity, substitute random numbers and see if it's true.
Try to spot inconsistencies, you're given x > 1/2 and y > 2/3, but you end up with the relationship x + y = 1.
Even if you do all the above, you will still make mistakes, we are all human. Be careful and try your best.